A silicon wafer passed through machining processes of block cutting, outer diameter grinding, slicing, and lapping, has a damaged layer, that is, an process damage layer, on the surface. In a manufacturing process of devices, since the process damage layer induces a crystal defect, such as slip dislocation, lowers mechanical strength of a wafer, and also affects adversely on electrical characteristic, it is required to be removed completely.
Conventionally, removing the process damage layer, which is introduced into a silicon wafer in the machining processes, by chemical etching, is carried out. As methods of the chemical etching, there are acid etching and alkali etching. Among these, the alkali etching has few problems in occurrence of a harmful by-product, such as NOx in the acid etching, and little danger when handling, and costs can also be reduced from the fields of material expense and equipment expense. Therefore, as an etching method, the alkali etching is preferable.
In the alkali etching of a silicon wafer, generally, an aqueous solution of sodium hydroxide is used. However, in the aqueous solution of sodium hydroxide for industry or for electronics industry, the present condition is that several ppm to tens ppm of metal impurities are included in the solution for industry and tens ppb to several ppm of metal impurities are included even in the solution of the grade for electronics industry.
As metal impurities included in the alkali aqueous solution, iron, nickel, chromium, and copper or the like, are given.
Therefore, as a material of an etching vessel, for example, even if it is the case that polypropylene is used in consideration of corrosion resistance, the silicon wafer is contaminated by the iron, nickel, chromium, and copper or the like, which are included in the alkali aqueous solution. In order to suppress that the metal impurities included in the alkali aqueous solution adhere to the silicon wafer in this way, the amount of metal ions dissolved in the alkali aqueous solution, is reduced by dissolving silicon in the alkali aqueous solution beforehand, by dissolving hydrogen gas (Japanese Patent Application Laid-open No. Hei 9-129624), by using an ion-exchange resin (Japanese Patent Application Laid-open No. Hei 9-129624), or by dissolving a dithionite or the like (Japanese Patent Application Laid-open No. Hei 10-310883), and methods for etching a silicon wafer by these solutions are proposed. However, even by these methods, the amount of contamination of the metallic elements adhering to or being diffused inside an etched wafer, is still a problem for manufacturing the latest high integration device. Therefore, its further reduction is required.
The present invention was made in view of the above-described problem. An object of the present invention is to provide an etchant and an etching method that contribute to prevention of metal contamination of a semiconductor silicon wafer, and a semiconductor silicon wafer in which metal contamination is extremely reduced.